This invention relates to a method for precision casting of clad optical components, and more particularly to the production of reinforced clad optical components, including complex clad optical shapes produced by casting plastic materials at a low cost and with a consistently high precision while at the same time avoiding the need for polishing or other surface treatments to the optical boundary. More particularly, the present invention is addressed to such a process for forming complex clad optical shapes wherein an extremely fine finish at the boundary surface is attainable without requiring an equally-fine finish upon the internal surface of a hollow shell that is used to form the component and employed, if desired, for reinforcement of the component.
An optical fiber or light pipe as it is sometimes referred to in the art, consists basically of a cylindrical core surrounded by a cladding. Both the core and the cladding are formed of materials possessing high optical transmittance at the desired wavelength. The core material has a refractive index slightly higher than the refractive index of the cladding material. The materials employed for this purpose may be solids, liquids or gases. Light is propagated along the length of the fiber or light pipe by total internal reflection in accordance with Snell's Law at the boundary between the core and the cladding. Absorption and scattering of light conducted by the optical fiber represent losses due to imperfections in the core material, inadequate smoothness at the boundary between the cladding and the core and by contaminants in the optical fiber material. Glass optical fibers are normally produced by a continuous drawing process at a carefully-controlled temperature. Crown or soda limeglass or other glass may be used to produce the glass optical fibers. On the other hand, plastic optical fibers have many desirable qualities and are normally produced by a continuous extrusion process. These are but a few examples of the type of optical fibers that are well known in the art and may be selected to form the core and/or cladding in the process of the present invention.
To provide a system employing light-conducting optical fibers, it is frequently necessary to utilize a variety of special-purpose transitions and fittings to adapt optical fibers to other optical fibers having different characteristics or to adapt optical fibers to other devices. Such special-purpose transitions and fittings are required to perform power-splitting functions, signal-combining functions or directional-coupling functions. It is to be understood that the expression "optical fibers" as used herein is intended to mean a single optical fiber or bundles of optical fibers as is appropriate to the description. When fabricating such transitions and fittings, the fabrication process is frequently complicated by requirements for varying cross sections and multiple branches. These requirements give rise to serious fabrication problems which the present invention is designed to overcome. Ligh-conducting components, such as transitions and fittings, must have a continuity of core material that is maintained throughout the component to prevent excessive loss due to reflections. This requires that the joints must be formed with great precision and then cemented or fused together with great care. Furthermore, the continuity of the cladding in the component must be maintained throughout to prevent unguided propagation or leakage at the discontinuities. Moreover, the boundary between the core and the cladding of a component must remain extremely smooth throughout the component to minimize scattering of light while propagated along the component. It is essentially impossible or at least excessively expensive to employ conventional techniques to fabricate light-conducting optical fiber components including fittings and transitions while meeting these requirements.
The novel process of the present invention includes the utilization of many modern plastics possessing excellent optical properties and providing, in addition, the ability to cast complex shapes at a relatively low cost. Many such plastics also maintain good optical properties over a very wide temperature range. It is to be understood, of course, that materials other than plastics may be employed to produce components with complex shapes for systems utilizing optical fibers by the novel process disclosed herein. When conventional techniques are employed to cast core material of an optical component, such as a fitting and transition, it is possible to apply a cladding of material having a different, i.e., lower, index of refraction by dipping or spraying the cladding material onto the cast core material. Such a process is relatively practical; however, the boundary surfaces either must be polished prior to cladding or the molds must be sufficiently smooth and free of fissures, particularly at parting surfaces of the mold so as to provide the necessary fine finish at the boundary surface.